The assignee of the present applicant is the registered proprietor of a number of patents and patent applications which disclose dielectrically-loaded antennas for operation at frequencies in excess of 200 MHz. Examples of such patents are GB2292638B, GB2310543B and GB2367429B. In each case, the antenna comprises an electrically insulative antenna core of a solid material having a relative dielectric constant greater than 5, a three-dimensional antenna element structure disposed on or adjacent the outer surface of the core and defining an interior volume, and a feeder structure which is connected to the element structure and passes through the core. Typically, the antenna element structure comprises conductive helical elements on a ceramic cylindrical core, the elements being arranged in pairs, each pair comprising diametrically opposed helical tracks plated on the cylindrical surface of the core. Each helical element extends from a radial connection to the feeder structure on a distal end surface of the core to a conductive sleeve which is connected to a shield conductor of the feed structure at a proximal end surface of the core, the sleeve thereby forming a balun so that, at an operating frequency of the antenna, the helical elements are provided with a substantially balanced feed point at the distal end surface.
Such an antenna, when provided with four helical co-extensive circumferentially spaced elements or groups of elements, has a mode of resonance which renders it especially suitable for receiving signals transmitted by earth-orbiting satellites, the signals being transmitted as circularly polarised waves. A particular use of such antennas, therefore, is for receiving signals transmitted by the Global Positioning System (GPS) satellite constellation.
The entire disclosure of the above-mentioned patents is incorporated in the present specification by reference.
There is a need for handheld mobile communication devices, such as mobile telephones or cellphones using terrestrial signals, also to receive signals from satellite systems such as the GPS constellation. Commonly, such mobile communication devices have a planar inverted-F antenna (PIFA) for transmitting and receiving terrestrial signals. A PIFA is a single-ended antenna in that it requires a conductive body to act as a ground plane for reflecting wave energy present on a radiator structure of the antenna so as to produce a standing wave. PIFA antennas may have at least one resonating finger which, at its base, is typically connected to a feed connection element connecting the radiator structure represented by the finger to a signal port of associated RF transmitting and receiving circuitry, and by a shunt element to a ground connection which is spaced apart from the signal port. The bandwidth of the antenna is determined, inter alia, by the width of the radiating finger and its spacing from the ground plane. The structure as a whole, i.e. the antenna and the associated conductive body, may be resonant in a number of different modes at different frequencies.
It has been found that if a dielectrically-loaded antenna such as those described in the above-mentioned patents is incorporated, together with a GPS receiver in a mobile telephone having a PIFA for transmitting and receiving terrestrial signals, severe breakthrough occurs between the PIFA and the GPS receiver when the mobile telephone transmitter is on. The degree of breakthrough depends on various factors including the frequency and bandwidth of the transmitted signal, the resonant characteristics of the PIFA, and the frequencies of the signals to be received by the dielectrically-loaded antenna and the associated receiver. In general, the breakthrough is such that there may be no useful signal reception via the dielectrically-loaded antenna when the mobile telephone transmitter is on.